Photocathode Studies @ FLASH: Quantum Efficiency (QE) · Production: diagnostic on photocathodes The photoemissive properties of produced •QE map @ 254nm cathodes are checked performing

Photocathode Studies

@ FLASH:

Quantum Efficiency (QE)L. Monaco

Work supported by the European CommunityWork supported by the European Community(contract number RII3-CT-2004-506008)

1/27WSHQE–October 4-6, 2006 L. Monaco INFN Milano – LASA

Main Topics

– Overview Photocathode Production– Production at LASA and transportation

h Ph h d D b– The Photocathode Database

– CW QE measurements (Hg lamp)– Experimental set up – Experimental set-up – Results of measurements at FLASH

– Pulsed QE measurementsQ– Laser energy calibration– Measurements on different cathodes

Results – Results – QE maps

– Conclusions


Quantum Efficiency

For the FLASH laser (λ = 262nm)QE(%) ≈ 0.5*Q(nC)/E(µJ)The design asks for 72000 nC/sThe design asks for 72000 nC/s

• QE required for FLASH:Q q f> 0.5 % to keep the laser in a reasonable limit: within an average power of ~WD i f t l t f • Design of present laser accounts for QE=0.5% with an overhead of a factor of 4 and has an average power of 2 W (IR)and has an average power of 2 W (IR)

• Cs2Te cathodes found to be the best choice


Photocathode Production: Preparation ChamberPhotocathodes are grown @ LASA on Mo plugs under UHV condition.g p gTransport Box Source holder

Preparation Chamber @ LASAMasking

Preparation Chamber @ LASA

Mo Plug• UHV Vacuum System - base pressure 10-10 mbar• 6 sources slot available• Te sources out of 99.9999 % pure element• Cs sources from SAES®

• High pressure Hg lamp and interference filter for online monitoring of QE during production


Masking• Masking system• 5 x UHV transport box

Production: diagnostic on photocathodesThe photoemissive properties of produced • QE map @ 254nmThe photoemissive properties of produced cathodes are checked performing spectral response measurements and QE maps (also at different wavelengths).

QE map @ 254nm(Hg lamp, interferential filter,

1mm spot diameter)

QE map:73-1i(254nm) of cathode #73 1 (23-03-2004) +/-4mm step 0 5mm

•Spectral response(Hg lamp, interferential filters, 5

6

7

8

9

QE map:73 1i(254nm) of cathode #73.1 (23 03 2004), / 4mm, step 0.5mm

5

6

7

8

1.E+01

1.E+02

g p3mm spot diameter)

10

15

20

5

10

15

200

1

2

3

4

1

2

3

4

1.E-02

1.E-01

1.E+00

QE

(%) 0

50

5

12

14

16

QE map:73-1i(254nm) of cathode #73.1 (23-03-2004), +/-4mm, step 0.5mm

6

7

8

Just after the d iti

1.E-04

1.E-03

2 2.4 2.8 3.2 3.6 4 4.4 4.8 5.2 5.6 6Photon Energy (eV)

6

8

10

12

2

3

4

5

d=5mm

deposition


Photon Energy (eV)

2 4 6 8 10 12 14 16

2

4

1

d=5mm

Production: from LASA to FLASH and PITZProduced cathodes are loaded in the Transport Produced cathodes, are loaded in the transport box and shipped to FLASH or PITZ keeping the UHV condition.

box

The box is then connected to the RF gun.

Since 1998, we have shipped to TTF h I FLASH d PITZTTF phase I, FLASH and PITZ:

•49 x Cs2Te

•2 x KCsTeK

•25 x Mo

•Total transfers from LASA: 25


RF gun and FLASH linac

Cathode in the RF Gun• Photocathode inserted into the gun backplane• Photocathode inserted into the gun backplane


Production:The Photocathode DatabaseMany of the data relative to photocathodes (production operation lifetimes) and transport box (production, operation, lifetimes) and transport box are stored in the “photocathode database” whose WEB-interface is available at:

http://wwwlasa.mi.infn.it/ttfcathodes/http://wwwlasa.mi.infn.it/ttfcathodes/The database keeps track of the photocathodes in the different transport boxes and in the different

labs (TTF, PITZ and LASA).( )


CW QE measurements: Experimental set-upTh i t l t f th CW QEThe experimental set-up for the CW QE

measurements is mainly composed by:

towards the RF gun

A high pressure Hg lampInterferential filters (239nm 254nm 297nm cathode

1.2 nA

300 V

towards theexchangechamber

(239nm, 254nm, 297nm, 334nm)

Picoammeter

Hg lamp

interferentialfilter

lensneutraldensity filters

cathodecarrier

cathodes

picoammeter

Power energy meter

Neutral density filters

Optical components (1 lens,

condenserpin-holepin-hole

anode

viewport2n W

transportpowermeter


1 mirror, 2 pin-holes) power headanodetransport

box

CW QE measurements: ResultsMeasured @ DESY on March 31 2006 Measured @ DESY on March 31 2006

Data have been fitted to evaluate: the QE @ 262nm and Eg+Ea

Cathode Dep. data QE@254nm (LASA)

Operation lif ti

QE@254nm (DESY)

QE@262nm (DESY)

Eg+Ea ( V)(LASA) lifetimes (DESY) (DESY) (eV)

73.1 23-Mar-05 7.9% 86 1.64% 0.79% 4.16572 1 22-Mar-05 9 2% 166 0 44% 0 33% 4 16872.1 22 Mar 05 9.2% 166 0.44% 0.33% 4.16823.2 16-Sep-04 7.2% 161 0.22% 0.15% 4.157

Cathode 73.1 Cathode 72.1 Cathode 23.2

100

101 Cathode 73.1 - Eg+Ea =4.1654eV - QE (@262 nm) =0.78732% - Slope =1.2417

← QE (@262 nm) =0.79%

ExperimentalFit

10-1

100Cathode 23.2 - Eg+Ea =4.157eV - QE (@262 nm) =0.15369% - Slope =1.6575

← QE (@262 nm) =0.15%

ExperimentalFit

10-1

100Cathode 72.1 - Eg+Ea =4.168eV - QE (@262 nm) =0.32676% - Slope =1.2422

← QE (@262 nm) =0.33%

10-2

10-1

QE

[%]

10-3

10-2

QE

[%]

10-2

QE

[%]

Experimental


4 4.2 4.4 4.6 4.8 5 5.2 5.410-3

photon energy [eV]

4 4.2 4.4 4.6 4.8 5 5.2 5.4

10-4

photon energy [eV]

4 4.2 4.4 4.6 4.8 5 5.2 5.4

10-3

photon energy [eV]

ExperimentalFit

CW QE measurements: Data Analysis• CW data analysis CW data analysis

– Fitting of the spectral response

( )[ ]mEEhAQE +⋅= ν ( )[ ]AG EEhAQE +−⋅= ν

where A is a constant, EG and EA are energy gap and electron affinity.

100

101 Cathode 73.1 - Eg+Ea =4.1654eV - QE (@262 nm) =0.78732% - Slope =1.2417

Q (@262 ) 0 9%

ExperimentalFit

An example is given for the l i f th CW QE d t

10-1

QE

[%]

← QE (@262 nm) =0.79% analysis of the CW QE data for cathode 73.1.In this case:

10-2

C th d #73 1

• Eg+Ea = 4.165 eV


4 4.2 4.4 4.6 4.8 5 5.2 5.410

-3

photon energy [eV]

Cathode #73.1

CW QE measurements: Data Analysis10

100

0.01

0.1

1

10

Just after the depositionQ

E (%

)

Photon energy (eV)2 3 4 5 6 7

1 .10 4

1 .10 3

the deposition

Powel(*)

1.E+00

1.E+01

1.E+02 Powel( )

(*)R. A. Powell, W. E. Spicer, G. B. Fisher, P. Gregory, “Photoemission studies of cesium telluride”, PRB 8 (9), p. 3987-3995 (1973)

1.E-02

1.E-01

QE

(%) A fresh cathode shows a Eg+Ea of

about 3.5 eV.

S l t f th

1.E-04

1.E-03just after the depositionafter 61h and 55minafter 114h and 42minafter 406 and 20min

Several measurements of the spectral response have been performed to study the Eg+Ea increase vs. time (maximum pressure f 10 9 b )


1.E-052 2.4 2.8 3.2 3.6 4 4.4 4.8 5.2 5.6 6

Photon Energy (eV)

of some 10-9 mbar)

Pulsed QE measurements: laser energy calibration experimental set-up

The laser energy transmission (from the laser hut to the tunnel)

polarizer movable mirror

laser hut to the tunnel) has been evaluated for different iris diameters (3.5mm, 2.0mm and 0 16mm) and different 0.16mm) and different energies.

Th l h

λ/2 plate

The laser energy has been measured using a Pyroelectric gauge (Joulemeter), varying the laser energy using the variable attenuator (λ/2 wave plate + polarizer).

Joulemeter mirror


Pulsed QE measurements: laser beamline transmission analysis

Th QE d h l d h l bl • The QE measurement procedure uses the laser energy measured on the laser table

• Transmission to the vacuum window is regularly measured

• Transmission of the vacuum window (92 %) andfl i i f h l i (90 %) d f

2.5O:\TESLA\TTF2\QE\31-03-06\2006-03-31T214932-QE.dat

0.15Mean trasnmission:14.1868 [%]above4000steps

reflectivity of the vacuum laser mirror (90 %) are accounted for

iris = 3.5 mm as an example:

0 5

1

1.5

2

Ene

rgy

[ μJ]

0.135

0.14

0.145La

ser r

oom

/ tu

nnel

Laser Room

sin2 fit Iris = 3.5 mm•Laser energy is measured as a function f h bl

2000 4000 6000 8000 100000

0.5

Attenuator Step

0.5O:\TESLA\TTF2\QE\energy calibration\cal-pyro-31032006_16512iris.txt

2000 4000 6000 8000 100000.13

Attenuator Step

Tunnel0.18

nel)

Mean trasnmission:17.1078 [%]above4000steps Iris = 2.0 mm

of the variable attenuator setting

•fitted by sin2

0.1

0.2

0.3

0.4

Ene

rgy

[ μJ]

sin2 fit

0.16

0.165

0.17

0.175

n2 (Las

er ro

om) /

sin

2 (tun

i i 16512

fitted by sinto evaluate the transmission


2000 4000 6000 8000 100000

Attenuator Step

2000 4000 6000 8000 10000

0.155

Attenuator Step

sin

iris =16512

Iris = 0.16 mm (σ)

Pulsed QE measurements: laser beam line transmission measurements

The laser beamline transmission has been evaluated four times (from March to August 2006) to take care of changes in the optical transmission path.

Iris Φ (mm) Iris (motor Date (tunnel Date (laser room Transmission Used steps) file) file)

3.5 16512 12-Mar-06 12-Mar-06 13.21 % From 12 March to 31

March2.0 17280 12-Mar-06 12-Mar-06 6.64 %0 16 18208 not done not done0.16 18208 not done not done -3.5 16512 31-Mar-06 31-Mar-06 17.10 % From 31

March to 6 June2.0 17280 31-Mar-06 31-Mar-06 8.75 %

0 16 18208 31-Mar-06 31-Mar-06 0 85%0.16 18208 31 Mar 06 31 Mar 06 0.85%3.5 16512 not done not done - From 6 June

to 7 Aug2.0 17280 6-June-06 6-June-06 7.18 %0.16 18208 not done not done -3.5 16512 not done not done - From 7 August

till now2.0 17280 7-Aug-06 7-Aug-06 4.49 %0.16 18208 not done not done -


Pulsed QE measurements: measurement analysis

[ ] [ ]The QE measurement is done following this procedure:

[ ] [ ] [ ][ ] 100JE

eVECQ100

nphnel%QE

cath

ph ⋅⋅

=⋅=1. Measurement of the charge (toroid T1, Q[C])

2.Measurement of the laser energy (laser hut) E

3.Calculation of the laser energy on the cathode E [J] i t i i ( id i th 262 nm: QE(%) ≈ 0.5*Q(nC)/E(µJ)Ecath [J] using transmission (considering the losses due to the vacuum window and mirror)

2.5

3O:\TESLA\TTF2\QE\14-03-06\2006-03-14T181811-QE.dat

QE =3.12%

6 nm QE(%) 0.5 Q(nC)/E(µJ)

1.5

2

ge (n

C)

The QE value is then obtained

fitting the charge trend @ low charge

0 5

1

Cha

rg

to be sure not to be affected by the space charge

fitting the charge trend @ low charge space charge

effect

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10

0.5

Laser Energy (uJ)

p g

The relative and systematic errors are in the order of 20 %.


The systematic error is mainly due to the uncertainty of identifying the linear part for the fit and due to the transmission measurement uncertainty

Pulsed QE measurements: cathode lifetimeQE of cathodes are measured

5

672.1

72 1frequently within months.

Example: cathode 72.1 and 73.1.3

4

5

E (%

)

mean 72.1CW_72.1

1

2

Q

End of lifetime QE < 0.5%

• We define the end of lifetime when the QE reaches 0.5 %

• The CW QE of cathode

473 1

014-Nov-05 14-Dec-05 13-Jan-06 12-Feb-06 14-Mar-06 13-Apr-06

date

• The CW QE of cathode 73.1 is compared with the pulsed QE measured the same day.

2.5

3

3.5

%)

73.1mean 73.1CW_73.1

y

• The difference may be explained considering the increase of the charge due

0 5

1

1.5

2

QE

(

End of lifetime QE < 0.5%

to the field enhancement.

• All cathodes show a drop of the QE over time, with


0

0.5

13-Mar-06 19-Mar-06 25-Mar-06 31-Mar-06 6-Apr-06

date

QQE m , wdifferent characteristics.

Pulsed QE measurements: drop of QE with time We can relate We can relate the drop of QEwith the vacuum condition in the RF gun.

3

3.5

473.1mean 73.1CW 73 1

• As an example, early 2006, the RF gun has been operated with 300 μs long RF pulses.

1.5

2

2.5

3

QE

(%)

CW_73.1p• Up to this, the pulse length

was restricted to 70 μs. • During the long pulse

operation period, the

0

0.5

1End of lifetime QE < 0.5%

operat on per od, the pressure increased from 5÷7·10−11 mbar to 2·10−10 mbar.

• This coincides with the drop of QE of cathode 73 1



dateof QE of cathode 73.1.

Pulsed QE measurements: cathode 78.1Referring to cathode 78.1, several measurements have been done during about 3 months (period: April 19 to July 11) months (period: April, 19 to July, 11).

Also this cathode shows a drop of th QE ti

•long pulse operation (increase of vacuum, ion-back bombardment??)

•different growth of the cathode during depositionthe QE vs. time. •different growth of the cathode during deposition

•damaging due to dark current coming from ACC135

78 1

• 78.1 just after the deposition25

3078.1CW_78.1mean 78.1

15

20

QE

(%)

• 78.1 during operation

0

5

10


03-Apr-06 23-Apr-06 13-May-06 2-Jun-06 22-Jun-06 12-Jul-06

date

Comparison between:Pulsed QE and CW QE measurements

4

5

6

72.1mean 72.1CW_72.1

The pulsed QE measurements of cathode 72.1 and 73.1 have been compared with the CW QE value @ λ = 262nm, evaluated

2

3

4

QE

(%)

3.5

473.1mean 73.1

from the spectral response.

0

1

14 N 05 14 D 05 13 J 06 12 F b 06 14 M 06 13 A 062

2.5

3

E (%

)

CW_73.1

14-Nov-05 14-Dec-05 13-Jan-06 12-Feb-06 14-Mar-06 13-Apr-06date

0.5

1

1.5

QE

CW QE @ 262nm


date

The CW QE respect to the pulsed QE value is lower:

thi b d t th hi h l ti fi ld th th d i


•this can be due to the high accelerating field on the cathode in pulsed QE measurements.

Pulsed QE measurements: QE vs. phase laser/RF gun•Measurements have been performed on two cathodes varying p y gthe laser/RF gun phase.

For cathodes 72.1 and 78.1,

the measured QE @ 70 deg is higher respect to the one measured @ 38 deg.

cathode #72.10.65

0 45

0.5

0.55

0.6

%)

cathode #78.1

25

26

0.3

0.35

0.4

0.45

QE

(%

iris = 3.5mmiris = 2mm

21

22

23

24

QE

(a.u

.)0.2

0.25

30 38 46 54 62 70 78RF phase

mean

18

19

20

21

iris = 3.5mmiris = 2mmmean


30 38 46 54 62 70 78

RF phase

Pulsed QE measurements: analysis (1)• RF data analysis – QE enhancement

– QE @ given acc. gradient Eacc and phase φ– with a given laser energy without space charge

( )m

E ⎤⎡ ( ) ( )acceeAG

EqqEEhAQE⎥⎥⎦

⎤

⎢⎢⎣

⎡

⋅⋅⋅⋅⋅

⋅++−⋅=04sin

επφβν

where E is the accelerating field φ is the phase RF/laser β is where Eacc is the accelerating field, φ is the phase RF/laser, β is geometric enhancing factor

Using the values calculated b f f E E d 1 6before for A, EG+EA and m, the geometric enhancing factor results:

β 101.2

1.4

1.6

QE

[%]

β= 10

with Eacc = 40.9 MV/m and the phase φ 38° from the

0.8

1

Q

cathode 73.1


the phase φ = 38° from the experimental measurement. 0 5 10 15 20 25 30 35 40 45 50

0.6

Eacc [MV/m]

• RF data analysis – Laser spot profile influencePulsed QE measurements: analysis (2)

– QE @ given Eacc and φ, at different laser energies• Space charge forces have to be taken into account and depends on the

laser transverse profile.1.51.5

11

Laser Beam Transverse Profile

Square profile Gaussian profile

0 5 .10 4 0.001 0.0015 0.002 0.00250

0.5

0 5 .10 4 0.001 0.0015 0.002 0.00250

0.5

Transverse Profile

Laser spot radius

2

2.5

3Laser spot radius

2

2.5

3

0 5

1

1.5

Char

ge

1

qc1 Eacc_gun φπ

180⋅,⎛⎜

⎝⎞⎟⎠π⋅ rbeam

2⋅

nC

1

1.5

2

Char

ge

1

qc1 Eacc_gun φπ

180⋅,⎛⎜

⎝⎞⎟⎠π⋅ rbeam

2⋅

nC

Extracted Chargevs.

Laser Energy


0 0.2 0.4 0.6 0.8 10

0.5

Laser Energy0 0.2 0.4 0.6 0.8 1

0

0.5

Laser Energy

Pulsed QE measurements: Comments to the analysis• The influence of the laser spot profile mainly affects the shape of the p p y p

charge vs. laser energy curves.• With this “simple” model, we can explain the shape of the curve and some

of the asymptotic values. It would be very helpful to have CW QE and pulsed QE measurements • It would be very helpful to have CW QE and pulsed QE measurements in the same day (QE constant) to further study the model.

3QE :3.23[%]; spot diameter:3.8586[mm]; laser sigma:0.1099[mm]

Example for cathode 73 1

2

2.5

3 Example for cathode 73.1

•Laser spot/iris diameter = 3.5mm.

1

1.5

Cha

rge

[nC

] •Extrapolated spot size = 3.8mm.•QE from the linear fit =

0 0 1 0 2 0 3 0 4 0 5 0 6 0 7 0 8 0 9 10

0.5

ExperimentalFit

QE from th n ar f t 3.1%•QE from this analysis = 3.23%


0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1Laser energy [μJ]

3.23%

Pulsed QE measurements: QE map (1)QE maps by scanning a small laser spot over the cathode

ti i i 0 16 ( ) t i 0 3 tiny iris = 0.16mm (σ), step size 0.3 mm.

Map of the charge itt d f th th d emitted from the cathode

moving the iris only.

cathode 73 1

Map of charge emitted from the cathode moving i is d i t th

cathode 73.1

iris and mirror together.

The photoemissive layer is 5 mm in diameter.

W ll p d d t Well reproduced, center position: (-0.2,-2.2) mm.

5 di f h


5mm diameter of the photoemissive layer

Pulsed QE measurements: QE map (2)QE maps cathode 77.1, used to center the laser beam on the cathode.

QE maps before alignment

QE map of cathode 77.1 after centering of the laser beam


Conclusion• CW QE measurements:CW QE measurements

– Experimental set-up in the FLASH tunnel has been installed– CW QE has been measured @ FLASH

Pulsed QE measurements:• Pulsed QE measurements:– Laser beamline transmission calibration– QE vs. time and vs. RF phases– Analysis of the pulsed QE measurements:

• Eacc, RF phase, etc.• QE mapsQ p

– Tool to check the centering between the laser spot and the photoemissive film

• For the futureFor the future– On-line measurements of the laser beamline transmission to

have “trustable” QE values and to “continuously follow” the cathode lifetime (helpful to decide when to change it, etc.)


( p g , )

Documents

Photocathode Studies @ FLASH: Quantum Efficiency (QE) · Production: diagnostic on photocathodes The photoemissive properties of produced •QE map @ 254nm cathodes are checked performing